The antibiotics Zithromax and Zmax SR (azithromycin) can cause potentially fatal irregular heart rhythms, Health Canada says.

Azithromycin is used for bronchitis, pneumonia, chest infections, urinary tract and other common infections.

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Pfizer is changing the precautions section of Zithromax's safety information. (Mark Lennihan/Associated Press)

Thursday's advisory from Health Canada says a "small absolute increase in the risk of cardiovascular deaths was observed in patients taking azithromycin as compared to those who took no antibiotics and those who took amoxicillin in a recent study."

In March, the U.S. Food and Drug Administration warned that the antibiotic, sold in Canada and the U.S. as Zithromax, can cause abnormal changes in the electrical activity of the heart that may lead to a potentially fatal irregular heart rhythm.

Health Canada said Pfizer reviewed data and decided to update the precautions section of the drug's monograph or safety information with more instructions.

The revised product label cautions using azithromaz when treating patients with risk factors such as:

  • A long QT interval — an abnormality of the heart's electrical system that predisposes you a very fast heart rhythm.
  • Low blood levels of potassium or magnesium.
  • A slower than normal heart rate.

The label also advises caution when patients are also using certain drugs known to prolong QT interval such as classes of arrhythmia medications, antipsychotic agents, antidepressants and fluoroquinolone antibiotics. 

Elderly patients may be more susceptible to drug-associated effects on the QT interval.